Higgs10: When spring 2012 turned to summer
By: Fabiola Gianotti& Joe Incandela
04 July, 2022 · Voir en français
In the sixth part of the Higgs10 series, the ATLAS and CMS experiments announce the discovery on 4 July 2012
At the Higgs boson search update seminar on 13 December 2011, things were already looking promising. The data had allowed us to constrain the Higgs boson mass to the range from around 115 to 130 GeV, and both ATLAS and CMS had tantalising hints of a new particle around a mass of 125 GeV. Those hints were not yet sufficiently strong to claim a discovery, the local significance was between 2.6 and 3.6 sigma, but they were enough to ensure that the eyes of the world would be on CERN as the data taking resumed in spring 2012 at a larger centre-of-mass energy: 8 TeV, compared with 7 TeV in 2011.
The year’s big high-energy physics conference, ICHEP, was to be held in Melbourne, Australia, starting on 6 July 2012. We both had our tickets booked, and an update on the Higgs boson search was a key part of the agenda. Plans were made for a two-way link to relay the Higgs boson sessions live from Melbourne to CERN’s Main Auditorium. Meanwhile, both experiments got on with data taking and analysis.
It was around mid-June that things started to get really interesting. By then, ATLAS had been seeing an excess of events in the two-photon channel, at the same mass as the excess reported at the end of 2011 based on an independent data sample, but nothing in the rarer four-lepton channels. It was clear to us both that we needed to see a signal in the gamma-gamma and lepton channels before going to the Director-General, Rolf Heuer. In the middle of June, CMS unblinded its analysis to find a four-sigma signal in the two-photon channel, and three-sigma in the leptons. Meanwhile, ATLAS’s Higgs boson sample received its first four-lepton candidates. We went to see Rolf.
The following weeks were incredibly intense. It was imperative that the collaborations maintained complete confidentiality, and it was impressive how well that was respected, not only outside but also within CERN. ATLAS did not know what exactly CMS had, and vice versa. The two of us would keep each other informed almost daily, but we did not disclose the other experiment’s results to our respective collaborations. Together with Rolf Heuer, we were the only ones who had the full picture of what was going on. This was essential to maintain confidentiality, but also to avoid ATLAS and CMS influencing each other, and to ensure that emotions did not affect the ongoing work. The pressure was enormous, people were working around the clock making millions of checks and cross-checks, and they had to remain calm and focused. The rest of CERN, and indeed anyone following particle physics, must have felt the energy emanating from the community, because the sense of anticipation was tangible.
The CERN Council met during the week of 18 June and decided that, whatever ATLAS and CMS had to say about the status of the Higgs boson searches, it should be said at CERN. We rapidly changed our travel plans, and CERN announced a Higgs boson update seminar for 4 July – the latest date compatible with both of us being in Melbourne in time for the plenary sessions of ICHEP. The primary direction of the two-way link with Melbourne was reversed: those arriving early for the conference would now follow the seminar at CERN remotely. The Council’s decision was taken as a sign that we had an announcement to make but, at that point, we were not telling anyone what we’d be announcing. Nevertheless, eminent theorists such as Carl Hagen and Gerry Guralnik decided to attend, and we invited all the other theorists who had been involved with developing the theory back in the 60s. As a result, François Englert and Peter Higgs also joined us on the day. Two years earlier, the four had shared the APS’s Sakurai Prize along with Robert Brout and Tom Kibble, both now sadly departed, for their work on spontaneous symmetry breaking in gauge theories.
It went right down to the wire. The results were still being checked and double-checked until just days before the seminar, and we were putting the final touches to our presentations until minutes before the seminar began. Walking into the auditorium, past the people rolling up their sleeping bags because they’d camped out overnight to ensure their places, we felt tremendous pressure along with great pride for what our community had managed to achieve over the decades. Then, as soon as it began, it seemed that a huge weight was lifted. The room was a sea of faces, ranging from those of people whose working lives had been devoted to building the LHC, ATLAS and CMS, to those whose careers were just beginning. Everyone was with us and, because the results were so compelling, neither of us needed our banks of back-up slides in case we were called upon to justify the details.
It was an amazing day, seen live around the world by half a million people, and reported by the media to over a billion. The media focused heavily upon us as the spokespersons but of course we were just the messengers. This success was the culmination of a multigenerational effort spanning decades. The capability of the particle physics community to deliver beyond expectation was truly inspiring. From the original theory through phenomenology to the design and construction of the accelerator, the detectors and the computing infrastructure, the tiny signal we were able to tease out from a large background was a credit to everyone who played a part. It was the triumph of a community that was able to achieve what many would have deemed impossible, bringing together expertise from every branch of the field.
Peter Higgs was treated like a rock star on the day. His reaction gives the measure of our field: when pressed for comment by the media, he replied that this was a day for the experiments, and there would be plenty of time to talk to the theorists later. A little over a year later, François Englert and Peter Higgs shared the Nobel Prize in physics for “the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, which recently was confirmed through the discovery of the predicted fundamental particle by the ATLAS and CMS experiments at CERN's Large Hadron Collider.”
Fabiola Gianotti and Joe Incandela, Spokespersons of the ATLAS and CMS experiments at the time of the Higgs boson’s discovery.